Bills and Island Songsters

The island effect has pre-occupied biological research for a long time. This led to the study of sexual dimorphism on islands, where there is less competition and so less selection against these dimorphisms. Song sparrows, Melospiza melodia, show a bill-size difference that is more pronounced on islands in California. The islands were the Channel and Coronado Islands.

Russell Greenberg and Raymond M. Danner from the Smithsonian Migratory Bird Center in Washington State publish this paper today in the Biology Letters of the Royal Society. The reduced species lists of many varied islands have always provided unique evolutionary studies for scientists. From Darwin to Attenborough, the tiny Galapagos to great Madagascar! Larger and smaller versions of mainland species evolve. Each of a group of islands provides interesting variation in the selection that is imposed on the animal or plant. In the case of birds, the bill often becomes larger in island species and varieties, as seen in these examples.

In order to investigate the 7 islands off the southern California coast, 1423 preserved skins from 6 of them and the mainland were studied. A greater bill dimorphism was found on the islands, although there were differences between the islands. Mainland bills tended to be similar to each other. Maximum temperature and size of island were judged to be the main factors responsible for the differences. The bill will have a function in heat loss for birds, so this is assumed to be a possible reason for the selection of the variations.

Large bills are associated with high temperature maxima and these are both found on the southernmost islands. On the other hand, temperature differences did not affect bill size on the mainland, where ecological differences must be different. A greater diversity of foraging is fund on larger islands and the mainland, which accounts for some selective pressure. Both sexes responded in equal degrees to the selective process. The male defends his territory in hot, exposed positions, possibly explaining his larger mouthpiece. The female is hidden away in her own micro-habitat, nesting or foraging in undergrowth.

Other endotherms must now be investigated to see if they can match the sexual dimorphism of these little island songsters. It will be interesting to see which species are chosen for investigation.

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